One type of imaging sensor utilizes Complementary Metal Oxide Semiconductor (CMOS) active pixels. Active Pixel Sensors (APSs) are utilized in various CMOS imaging devices, such as light sensors, telescopes, digital cameras, and video recorders. An APS captures an image of a scene of interest or detects light by converting incident light from the scene into electrical signals in an analog form.
A typical APS has an array of pixels or discrete regions, each pixel containing a light-sensitive element. The light-sensitive element can be a variety of elements including a photodiode, phototransistor, or other suitable device. Each light-sensitive element in a pixel generates a separate electrical current, which is proportional to the intensity of the incident light on that element. The varying magnitude of this electrical current is used as a basis for conversion into a stream of digital image data by an analog-to-digital converter (ADC). The digitized image data from all the pixels can then be displayed in a variety of formats, such as a composite image on a monitor or printed onto a sheet of paper. The digitized image data can also be analyzed for information concerning the properties of objects in the scene.
Many imaging sensors currently being designed include pixels having a photodiode. Photodiodes have certain limitations that must be accounted for when they are used in pixels in an APS. All photodiodes typically suffer from dark current, which is a leakage current from the photodiode when no light is striking the photodiode. Dark current occurs at any temperature above absolute zero and is temperature sensitive. As the temperature of the photodiode increases, the dark current increases as well. The amount of dark current also depends upon the material used to make the photodiode. In some APSs in certain light conditions, the dark current can be of the same order of magnitude as the current induced by photons striking the photodiode, thereby making the current induced by the photons indistinguishable from the dark current.
For example, because of dark current limitations, APSs for sensing infrared light are typically manufactured using indium gallium arsenide (InGaAs). InGaAs is typically used because it has a relatively high quantum efficiency and a relatively low dark current, but InGaAs has several disadvantages. InGaAs is relatively expensive compared to other semiconductor materials, such as germanium. InGaAs is also not as readily available as other semiconductor materials. In addition, use of InGaAs produces a relatively low yield. In the past, materials such as germanium and others, which are less expensive, readily available, and result in higher yields than InGaAs, have not been used to manufacture APSs for certain applications, such as for sensing infrared light. The excessive leakage current at room temperature of sensors made from these materials would render the sensors useless for sensing infrared light.
Therefore, there is a need for APSs that can compensate for leakage current in photodiodes.